In the field of telecommunications, i.e., cellular and mobile phones, it is desirable to minimize the power necessary to transmit a signal. One popular approach is to transmit fewer bits over a given period of time during which there is no speech. The IS-95 Direct Sequence Code Division Multiple Access (DS-CDMS) system is one example of a system that allows the speech encoder rate to be reduced when the speech activity in the transmitter is low. IS-95 has become a U.S. digital cellular standard. A more detailed description of IS-95 can be found in TIA/EIA Interim Standard 95: Mobile Station-Base Station Compatibility Standard For Dual Mode Wideband Spread Spectrum Cellular Standard, (July 1993).
In the IS-95 forward traffic channel, i.e., from the base station to a mobile station, the speech information is transmitted at one of two data rates, either 8 kbps or 13 kbps, depending on the speech encoder. Focusing on the 13 kbps speech encoder each frame of data, or data packet, consists of 267, 125, 55 or 21 information bits corresponding to an information bit sub-rate of 13.35 kbps (sub-rate R.sub.1), 6.25 kbps (sub-rate R.sub.2), 2.75 kbps (sub-rate R.sub.3) or 1.05 kbps (sub-rate R.sub.4), respectively. The information bit rate varies from packet to packet depending on the speech activity. After adding a variety of bits to each packet, the data packets are then encoded by a rate 1/2 convolutional (channel) encoder and the encoded data are repeated twice, four times and eight times for the frames with 6.25 kbps, 2.75 kbps and 1.05 kbps, respectively so that every frame has 576 symbols. To ensure a 19.2 kbps transmission symbol rate with 384 symbols per frame for each of the four sub-rates, the same communication rate used with the 8 kbps speech encoder, two out of every six bits in each frame are punctured.
Since the receiver detects a constant symbol rate, the Viterbi decoder that decodes the data packets does not initially know which of the four sub-rates was used in the transmitter for any given data packet. One way to determine the correct sub-rate at the receiver is to use the Viterbi decoder to decode the packet according to each of the four possible encoding methods, and then select the decoded data packet that has the least number of errors.
The conventional method decodes the received sequence four times, using the Viterbi decoder, and then compares each bit in the decoded sequence for each sub-rate to the sign of the corresponding symbol in the received sequence. The sub-rate which corresponds to the decoded sequence having the least number of bit errors is selected. This method is computationally intensive. An alternative method is described and claimed in the related application referenced above. That method relies on recognizing patterns of repeated symbols to determine the sub-rate. The task of identifying sub-rates by recognizing repetitive patterns is complicated with speech encoder receivers that involve puncturing. Since symbols are removed from the data packet, the repetitive patterns are disturbed.